Dear mingming, you can find a lot of information on secant varieties in Harris's book Algebraic Geometry, A First Course (Springer GTM 133), essentially presented as a set of thoughtfully conceived exercises. The ultimate reference on the subject is ZAK's monograph

http://books.google.com/books?id=0-BxhMVJvMsC&printsec=frontcover&dq=zak&lr=&hl=fr&cd=16#v=onepage&q&f=false

For those who don't know the concept let me briefly outline its basic idea. Given a $d$-dimensional variety $X$ in $\mathbb P^n$, take all the chords joining two points ( maybe not distinct...) of $X$ and consider the union $Sec(X)$ of these chords.This variety has dimension at most $2d+1$ and  generically you have equality.This allows for many nice very geometric constructions, for example by projecting from a point outside $Sec(X)$ .
You can easily show this way that every projective smooth variety of dimension $d$ embeds in $ \mathbb P^{2d+1}$ .